Specifically, if you place two uncharged metal plates side by side in a vacuum, they will move towards each other, seemingly without reason. They won't move a lot, mind. Two plates with an area of a square metre placed one-thousandth of a millimetre apart will feel a force equivalent to just over a tenth of a gram.

The Dutch physicist Hendrik Casimir first noted this minuscule movement in 1948. "The Casimir effect is a manifestation of the quantum weirdness of the microscopic world," says physicist Steve Lamoreaux of Yale University.